New images reveal never-before-seen structures of the solar
wind as it travels toward and impacts Earth

Newly reprocessed archival data from STEREO-A/SECCHI show details of the first Earth-directed
coronal mass ejection (CME) of the STEREO mission, from inception on December 12, 2008, to Earth impact on December 15,
2008. New processing enables following the details of the CME with the wide-field heliospheric imager cameras, out to
impact with the Earth 93 million miles from the Sun.
Courtesy SwRI/NASA

Boulder, Colo. — Aug. 18, 2011 — Using data
collected by NASA's STEREO spacecraft, researchers at
Southwest Research Institute® (SwRI®) and the National Solar Observatory have developed the first
detailed images of solar wind structures as plasma and other particles
from a coronal mass ejection (CME) traveled 93 million miles and impacted
Earth.

The images from a December 2008 CME event reveal an array
of dynamic interactions as the solar wind, traveling at speeds up to a
million miles per hour, shifts and changes on its three-day journey to
Earth, guided by the magnetic field lines that spiral out from the Sun's
surface. Observed structures include the solar wind piling up at the
leading edge of a CME, voids in the interior, long thread-like structures,
and rear cusps. Quiet periods show a magnetic disconnection phenomenon
called a plasmoid, "puffs" that correlate with in-situ density
fluctuations, and V-shaped structures centered on the current sheet — a
heliospheric structure in which the polarity of the Sun's magnetic field
changes from north to south.

"For the first time, we can see
directly the larger scale structures that cause blips in the solar wind
impacting our spacecraft and Earth," said SwRI's Dr. Craig DeForest, lead
author of an Astrophysical Journal article released online yesterday.
"There is still a great deal to be learned from these data, but they are
already changing the way we think about the solar wind."

"For 30 years," said co-author Dr. Tim Howard, also of SwRI, "we
have been trying to understand basic anatomy of CMEs and magnetic clouds,
and how they correspond to their source structures in the solar corona. By
tracking these features through the image data we can establish what parts
of a space weather storm came from which parts of the solar corona, and
why."

The team used a combination of image processing techniques
to generate the images over a distance of more than 1 AU (astronomical
unit), overcoming the greatest challenge in heliospheric imaging, that of
extracting faint signals amid far brighter foreground and background
signals. Small "blobs" of solar wind tracked by the team were more than 10
billion times fainter than the surface of the full Moon and 10 thousand
times fainter than the starfield behind them.

"These data are like the first demonstration weather satellite images that revolutionized
meteorology on Earth," said DeForest. "At a glance it is possible to see
things from a satellite that cannot be extracted from the very best
weather stations on the ground. But both types of data are required to
understand how storms develop."

In particular, the new images
reveal the shape and density of Jupiter-sized clouds of material in the
so-called empty space between planets; in contrast, in-situ probes such as
the WIND and ACE spacecraft reveal immense detail about the solar wind, at
a single point in space.

Funding for this research was provided by
the National Science Foundation SHINE Competition, the NASA Heliophysics
Program and the National Solar Observatory by the U.S. Air Force under a
Memorandum of Agreement. The paper, "Observations of Detailed Structure in
the Solar Wind at 1 AU with STEREO/HI-2," by DeForest, Howard and S.J.
Tappin (National Solar Observatory) was published online yesterday for the
September 1, 2011 print issue of the Astrophysical Journal.

STEREO is part of NASA's Solar Terrestrial Probes Program in NASA's Science
Mission Directorate in Washington. The program seeks to understand the
fundamental physical process of the space environment from the Sun to
Earth and other planets.